Resize an image in mm for labels, stickers and document forms: they state their size in millimetres, and a file only has pixels. This page converts at 300 DPI, opens the editor on 50 × 50 mm, shows the rounding, and saves the whole-pixel size. Part of the image resizer.
Resize image in mm: millimetres to pixels at 300 DPI
1 mm at 300 DPI
11.81 pxcomputed
50 × 50 mm
591 × 591 px, the size this page opens oncomputed from 590.55
The formula
pixels = mm ÷ 25.4 × DPIWikipedia, Pixel density
A DPI tag in a file
metadata only: it changes no pixelWikipedia, Dots per inch
Output format
– / –
Drop images or browse
JPEG · PNG · WEBP · AVIF · HEIC · up to 50 at once · max 30 MB each
HEIC
PNG
JPEG
AVIF
WEBP
Advanced settings1 on
SmallerAutoBetter
Auto tunes each image.
A hard cap for the number your form demands.
KB
A floor for forms that refuse small files.
Max resolution
×
Empty side keeps the ratio. Never enlarges.
Presets
Every named size, one editor
The same editor, the size already pressed. Pick the document or the platform.
Source: photo.jpg, 576.8 KB at 1920×1280, through the preset this page opens on (the centre cropped to the ratio and scaled to the size), then the engine at its defaults. The row is the real output.
Every byte count is the size of the actual file our engine produced from the named source, the same bytes you would download, not an estimate. The reference photo is one of our own photographs, an iguana on a branch, and the camera photo is a real one too. Drop your own file into the tool above to rerun any of them. How we measure names the sources and the settings.
Answered
Why does a millimetre size need a DPI before it becomes pixels?
Because a pixel is not a length.
It becomes one only when a printer or a screen lays it down at a density, so many pixels to the inch, and a millimetre size names no density. At the 300 DPI print convention a millimetre is 11.81 pixels; at 150 it is half that, and the same file is twice the size on paper. The formula is millimetres divided by 25.4, times the DPI, and the 25.4 is exact: the inch has been defined as 25.4 mm since the international yard agreement. This page assumes 300, the print convention, and says so beside every number. If a printer gives you another density, its number wins.
The other half of the truth is about the file. A DPI value inside a JPEG or PNG is metadata: a note about the intended print size, changing no pixel. A portal that checks dimensions counts the pixels, not the note. So the job is never “set the DPI”; the job is to export the right number of pixels, and the rest of this page is about that.
Computed
How many pixels are 35 × 45, 50 × 50, 50 × 70 and 100 × 150 mm at 300 DPI?
The sizes forms and labels write, computed here by the arithmetic the editor runs.
The middle column is the figure before rounding, which no millimetre size reaches as a whole pixel; the editor rounds each side on its own and exports the whole number. The named document frames have their own pages with the rules; this table is the arithmetic only.
Size
Pixels at 300 DPI
Before rounding
Ratio
35 × 45 mm
413 × 531
413.39 × 531.50
7:9
50 × 50 mm
591 × 591
590.55 × 590.55
1:1
50 × 70 mm
591 × 827
590.55 × 826.77
5:7
100 × 150 mm
1181 × 1772
1181.10 × 1771.65
2:3
Computed at render time from pixels = mm ÷ 25.4 × 300, each side rounded to the nearest whole pixel. The rounding is under half a pixel a side, under a twentieth of a millimetre on paper.
Answered
Why is the pixel count not a whole number, and does the rounding matter?
It does not matter, and here is the size of it.
50 mm at 300 DPI is 590.55 pixels; the editor exports 591. The Custom size box opens in mm with that size in force, and its fields read 50.04, because 591 whole pixels are 0.04 mm over the size you asked for. That is the rounding, shown rather than hidden. On paper it is a few hundredths of a millimetre, well inside what any cutter or frame allows. In pixels it is the whole number a portal wants to see, where a fraction would fail the check. Type 50 into that field and Apply stays grey: 50 mm rounds to the same 591, so there is nothing to change.
Answered
How do I resize an image in millimetres with the editor?
The Custom size box on the Crop tab opens in millimetres, with decimals allowed, so a template that says 12.5 × 12.5 goes in as written and the line under the fields answers 148 × 148 px at 300 DPI.
The readout at the top of the panel shows the size in force in pixels, 591 × 591 on arrival. The unit menu under the two fields switches to px, cm or in when a form states those. The link icon between the fields keeps the shape while you type; tap it to unlink when the new size has another shape, and type both sides. Press Apply, and the frame takes the new shape at the new pixel size. The rounding happens at that moment, once, on the pixels.
Answered
My photo is not square: crop it or fit it into 50 × 50 mm?
Crop for a document photo, Fit for a label.
The 1:1 preset opens with the largest centred square drawn on the picture, 1280 × 1280 of our 1920 × 1280 reference photo, and Crop keeps that square, dragged to wherever the face or the object sits. Fit keeps the whole picture inside the square and fills the two bands with a colour, white unless you pick another, so a wide product shot lands on a square sticker with nothing cut off. Both export exactly 591 × 591 pixels. Neither stretches. A document portal wants the face filling the frame, so Crop is the one for a form, and the passport pages carry each frame’s rules.
File size
Does the file get smaller at 50 × 50 mm?
Yes, by 90 percent on our reference photo, and most of that is the encoder, not the millimetres.
The 1920 × 1280 source weighs 576.8 KB. Saved at full size at the engine’s Auto quality it is 273.9 KB, 53 percent smaller with every pixel kept. As the 591 × 591 square it is 57.4 KB: the square left out the sides of the picture and scaled the rest down to 46 percent of its side. A millimetre size is small in pixels, so the file that meets it is small too. When a form also caps the bytes, set Max file size in Advanced settings and the export lands under it in the same save.
Treatment
Pixels
Result
vs source
Reference photo, as uploaded
1920×1280
576.8 KB
0%
Saved at Auto, full size
1920×1280
273.9 KB
−53%
50 × 50 mm, cropped square
591×591
57.4 KB
−90%
Source: photo.jpg, our reference photo, 576.8 KB at 1920×1280. The second row is the same photo saved at Auto with no size change; the third is this page's preset, the centre square scaled to 591 × 591 and saved at Auto. Each row is the file the engine produced.
Be sure first
When millimetres are the wrong unit
The printer wants 150 or 600 DPI?
The box converts at 300 only. Divide millimetres by 25.4, multiply by their DPI, and type the pixels with the unit on px.
The form states pixels?
Type pixels and stop converting. A form that gives both units is describing the pixels.
A screen or a slide?
No unit applies but the pixel. The 1920 × 1080 page opens on the Full HD frame.
A passport or ID frame with rules?
The passport pages open on each frame with its rules and the portals that state them, sourced. This page has the arithmetic and no rules.
The size is already pressed above. One drop settles most of these.
Q:How do I resize a photo in millimetres?
Drop it in. The editor opens on Crop with 50 × 50 mm in force, 591 × 591 pixels at 300 DPI. For your own size, the Custom size box is already in millimetres: unlink the sides if the shape differs, type the width and the height, decimals allowed, and press Apply. The line under the fields prints the pixels before you commit. Place the frame, or press Fit, then Save.
Q:How many pixels is 50 × 50 mm at 300 DPI?
591 × 591 pixels. Fifty millimetres divided by 25.4 and multiplied by 300 is 590.55, which no printer can lay down, so the editor rounds each side to the nearest whole pixel. The export is 591 × 591, and read back into millimetres that is 50.04 mm, 0.04 mm over the size you asked for, a difference no cutter or frame can show.
Q:Is 35 × 45 mm the same as 3.5 × 4.5 cm?
Yes, one size written two ways. A centimetre is ten millimetres, so 35 × 45 mm and 3.5 × 4.5 cm both become 413 × 531 pixels at 300 DPI, and the editor exports the same file whichever unit you type. The rules for that frame and the portals that ask for it live on the passport pages, with their sources.
Q:What DPI do I need for a photo stated in millimetres?
300 DPI, the density the editor converts at, unless the form or the printer names another. A millimetre size only becomes pixels once a density is chosen, and 300 is the print convention, about the point past which the eye stops seeing more detail at reading distance. A form that states millimetres and pixels together has already done the conversion; type its pixels and treat the millimetres as the description.
Q:Why is the pixel count not a whole number?
Because 300 pixels to the inch is 11.81 pixels to the millimetre, and few millimetre sizes multiply that into a whole number. 50 mm is 590.55 pixels; 35 mm is 413.39. A file can only hold whole pixels, so each side is rounded on its own. The rounding is always under half a pixel, which is under a twentieth of a millimetre on paper.
Q:Can I resize in mm without cropping my photo?
Yes. Once the size is in force, press Fit on the Crop / Fit switch. The whole picture is scaled to sit inside the millimetre size and the rest is a fill colour: white by default, or black, grey, green or any colour from the well. The export is still exactly 591 × 591 pixels for 50 × 50 mm, so a square label prints from a wide photo without losing its edges.
Q:Does the tool write the DPI into the file?
Do not count on a tag; count on the pixels. The export is sized in whole pixels, which is what a portal checks and what a printer lays down at the size you order. A DPI value inside a file is metadata that changes no pixel. We make no promise about that tag in the export. If a form names a DPI beside the pixels, meet the pixels; the DPI describes how they print.
Q:Does the photo resizer in mm also take cm and inches?
Yes, the same unit menu offers cm and in, and all three convert at 300 DPI. Use whichever unit the form or the order is written in, and the export is the same whole-pixel file: 10 cm and 100 mm both become 1181 pixels. The centimetre page opens on 10 × 15 cm with the lab sizes, and the inch page on 4 × 6 in with the print sizes.
Q:Can I type a decimal size in millimetres?
Yes. With mm selected the fields take decimals, so a template that says 12.5 × 12.5 mm goes in as written and becomes 148 × 148 pixels at 300 DPI. Rounding happens once, at Apply, on the pixel count and never on the millimetres you typed. The line under the fields shows the whole-pixel result while you type, so you can check it before you commit.
Q:Will resizing in mm make the file smaller?
Usually, yes, because a millimetre size is small in pixels. Our 1920 × 1280 reference photo weighs 576.8 KB; as a 591 × 591 square it came back at 57.4 KB, 90 percent smaller. Most of that is the encoder, which saves the full-size photo at 273.9 KB on its own; the rest is the pixels the square left out.
Q:Which photo sizes are written in millimetres?
Document photos, labels, stickers and the small print sizes. Forms write 35 × 45 mm, 50 × 50 mm or 50 × 70 mm; a label sheet or a sticker template writes its cell in millimetres; a lab may write 100 × 150 mm for the print a centimetre order calls 10 × 15. The named document frames each have a page with their rules; the millimetre box takes any number.
Q:Do you keep my photo after I resize it?
No. We keep nothing: files are processed and wiped automatically, never archived and never used for anything else. The export leaves without its EXIF and GPS, so a photographed form or a document photo carries no record of the room it was taken in. No account is needed, nothing is watermarked, and the resize is free.
